This invention relates to a method for preparing methacrylic acid. More detailedly, this invention relates to a method for preparing methacrylic acid by catalytically vapor phase oxidizing isobutane.
As to a method for preparing methacrylic acid via methacrolein by catalytically vapor phase oxidizing an unsaturated hydrocarbon or tertiary alcohol such as isobutene or tertiary butanol, various methods have been proposed and some of them are industrially adopted.
Also as to a method for preparing methacrylic acid directly by catalytic vapor phase oxidation of isobutane as a saturated hydrocarbon, reports have been made in several publications including Japanese Laid-open Patent Publication No. 55-62041. Many of the methods use a hetero poly acid catalyst containing silicon, phosphorus or arsenic as the hetero atom and molybdenum and/or vanadium as the poly atom as disclosed in Japanese Laid-open Patent Publication No. 62-132832, Japanese Laid-open Patent Publication No. 63-145249, etc. As catalysts other than hetero poly acid catalysts, Moxe2x80x94Vxe2x80x94Sbxe2x80x94Texe2x80x94O catalysts (Japanese Laid-open Patent Publication No. 9-278680 and Japanese Laid-open Patent Publication No. 10-128112), Pxe2x80x94Vxe2x80x94Au and/or Ag catalysts (Japanese Laid-open Patent Publication No. 5-178774), etc. have been reported.
However, such prior catalysts have the disadvantage that when the conversion of isobutane is fixed at a low level, the selectivities of methacrylic acid and methacrolein are high to some degree, but when the conversion of isobutane is fixed at a high level, the selectivities of methacrylic acid and methacrolein are lowered, and by-products such as acetic acid and acrylic acid and combustion products such as carbon monoxide and carbon dioxide increase.
The object of the invention lies in providing a method for preparing methacrylic acid in high selectivity and high yields by vapor phase oxidizing isobutane in the presence of a catalyst using molecular oxygen or a molecular oxygen-containing gas.
The present inventors have found that when, in preparing methacrylic acid by catalytic vapor phase oxidation of isobutane, a catalyst comprising a sparingly water-soluble salt of a hetero poly acid and a composite oxide containing phosphorus, molybdenum and vanadium is used, the above object can be accomplished, and have completed the invention based on the finding.
Thus, according to the invention is provided a method for preparing methacrylic acid which comprises, in preparing methacrylic acid by vapor phase oxidizing isobutane in the presence of a catalyst using molecular oxygen or a molecular oxygen-containing gas, using as the catalyst a catalyst comprising
(i) a sparingly water-soluble salt of a hetero poly acid and
(ii) a composite oxide containing phosphorus, molybdenum and vanadium.
The sparingly water-soluble of a hetero poly acid as component (i) of the catalyst of the invention means a salt of a hetero poly acid with at least one element selected from potassium, rubidium, cesium and thallium. The hetero poly acid is a generic term including typical hetero poly acids such as 12-molybdophosphoric acid, 12-tungstophosphoric acid, 12-molybdosilicic acid, 12-tungstosilicic acid, 12-molybdoarsenic acid and 12-tungstoarsenic acid; these hetero poly acids wherein part of molybdenum and/or tungsten is replaced with vanadium; or these hetero poly acids wherein molybdenum, tungsten or vanadium is coordinated in a mixed state. Such a sparingly water-soluble salt of a hetero poly acid can readily be prepared by adding to an aqueous solution of a hetero poly acid an aqueous solution containing at least one element selected from potassium, rubidium, cesium and thallium. In the invention, the thus obtained sparingly water-soluble salt of a hetero poly acid can be used in a slurry state as the obtained form or in a solid state after the separation of water from the slurry by a treatment such as filtration or centrifugation.
As the hetero poly acids, ones on the market can be used as such or after purification by a known method such as ether extraction, drying or recrystallization. It is also possible to use a hetero poly acid prepared by a known method, for example by acidifying with hydrochloric acid or the like an aqueous solution containing a sodium salt of a metallic acid such as molybdic acid or tungstic acid and sodium phosphate or sodium silicate or the like, heating the solution to make reaction occur, separating the reaction product with an ether as an etherified product from the resulting aqueous solution, and drying the product.
Among the sparingly water-soluble salts of hetero poly acids, preferably used are those having the element composition (but excluding oxygen) represented by the general formula (1), namely
AaBbMOcWdVexe2x80x83xe2x80x83(1)
(wherein A represents a counter cation and is at least one element selected from potassium, rubidium, cesium and thallium, B represents a hetero atom of the hetero poly acid and is at least one selected from silicon, phosphorus and arsenic, Mo, W and V represent molybdenum, tungsten and vanadium respectively which are poly atoms of the hetero poly acid, a, b, c, d and e represent an atomic ratio of the respective elements and when b is 1, a is 3 to 4, c is 0 to 12, d is 0 to 12 and e is 0 to 3, and the total of c, d and e is 12.).
The other component (ii) of the catalyst of the invention is a composite oxide containing phosphorus, molybdenum and vanadium as indispensable components, and it can further contain as other optional components potassium, rubidium, cesium, thallium, magnesium, calcium, strontium, barium, manganese, chromium, iron, cobalt, nickel, copper, zinc, aluminum, tin, lead, antimony, bismuth, yttrium, zirconium, titanium, niobium, tantalum, rhodium, palladium, platinum, silver, lanthanum, cerium, praseodymium, neodymium, etc.
Among them, preferably used are composite oxides represented by the following general formula (2)
PpMoqVrXsOtxe2x80x83xe2x80x83(2)
(wherein P, Mo, V and O represent phosphorus, molybdenum, vanadium and oxygen, respectively, X represents at least one element selected from potassium, rubidium, cesium, thallium, magnesium, calcium, strontium, barium, manganese, chromium, iron, cobalt, nickel, copper, zinc, aluminum, tin, lead, antimony, bismuth, yttrium, zirconium, titanium, niobium, tantalum, rhodium, palladium, platinum, silver, lanthanum, cerium, praseodymium and neodymium, and p, q, r, s and t represent an atomic ratio of the respective elements and when q is 12, p is 1 to 2, r is a number of 2 or less not including 0, s is 0 to 2 and t represents a number of oxygen atoms necessary for satisfying the valences of the respective elements.).
Preparation methods of the composite oxide are not particularly limited, and it can be prepared according to a conventional method. For example, it can be prepared by a method which comprises dissolving or dispersing such raw materials as mentioned below, heating the solution or dispersion to concentrate it to dryness. As phosphorus sources as a raw material, there can be mentioned orthophosphoric acid, pyrophosphoric acid, phosphorous acid, polyphosphoric acid, phosphorus pentoxide, phosphorus pentachloride, sodium phosphate, etc. As molybdenum sources, there can be mentioned molybdenum trioxide, molybdic acid, ammonium paramolybdate, sodium molybdate, etc. As vanadium sources, there can be mentioned, besides vanadium oxide, pentavalent, tetravalent or trivalent vanadium-containing compounds such as metavanadic acid or its salts, pyrovanadic acid or its salts, and vanadium oxyhalides. Hetero poly acids comprising phosphorus, molybdenum and/or vanadium such as molybdophosphoric acid and molybdovanadophosphoric acid can also be used as raw materials.
As raw materials of potassium, rubidium, cesium, thallium, magnesium, calcium, strontium, barium, manganese, chromium, iron, cobalt, nickel, copper, zinc, aluminum, tin, lead, antimony, bismuth, yttrium, zirconium, titanium, niobium, tantalum, rhodium, palladium, platinum, silver, lanthanum, cerium, praseodymium and neodymium as the optional components, there can be mentioned oxides, nitrates, sulfates, carbonates, organic acid salts, alkoxides, organic complex compounds of the respective elements.
There is no particular limitation about methods for preparing the catalyst of the invention, but for example, the following method can be adopted:
(1) a method which comprises separately preparing in advance a sparingly water-soluble salt of a hetero poly acid and a composite oxide containing phosphorus, molybdenum and vanadium and mixing them, or
(2) a method which comprises mixing an aqueous solution of a hetero poly acid with an aqueous solution containing at least one element selected from potassium, rubidium, cesium and thallium to prepare a slurry containing the resulting sparingly water-soluble salt of the hetero poly acid, adding to this slurry an aqueous solution separately prepared by dissolving in water raw materials containing phosphorus, molybdenum and vanadium and if necessary optional components, respectively, concentrating the resulting slurry under heating at 50 to 90xc2x0 C. and stirring to dryness, drying the resulting solid, and calcinating the solid at 300 to 500xc2x0 C. in an oxygen-containing gas.
Among them, the method of (2) is preferably used.
As to the ratio between component (i) and component (ii) in the catalyst of the invention, it is preferred that the molar ratio [AaBbMocWdVe]/[PpMoqVrXsOt] is 1:10 to 10:1, particularly 1:5 to 5:1.
The catalyst of the invention can be used not only in a powdery state, but also in such a state that it is molded alone or together with a carrier such as silica, alumina, titania, zirconia, silicon carbide or ceramics, or it is deposited on such a carrier. As to molding of the catalyst, there is no particular limitation, and it can be molded into an any shape such as spheres, cylinders, rings, arches or saddles by a so far known molding method such as tableting molding or extrusion molding. In the molding, there can be used, as a molding aid, inorganic material such as silica sol, alumina sol, talc or graphite, or organic material such as fatty acid salts. Inorganic fibers can also be used.
In the method of the invention, methacrylic acid can be prepared by vapor phase oxidizing isobutane in the presence of the above catalyst.
Isobutane may contain a small amount of n-butane, butenes, propane, pentane, etc. As the oxygen source used in the vapor phase oxidation, molecular oxygen or a molecular oxygen-containing gas can be used. Specifically, air is preferably used, but pure oxygen alone or diluted with an inert gas such as nitrogen, helium or carbon dioxide can also be used. Usually, the isobutane concentration in the raw material gas is 0.5 to 80% by volume, preferably 1 to 50% by volume, and the volume ratio of oxygen to isobutane is 0.05 to 20, preferably 0.1 to 4. Steam can be made to exist in the raw material gas, and in that case, it is preferred to make steam exist in an amount of 0.1 to 5 volumes of isobutane. When the catalyst is used in a fixed bed, the space velocity (STP) is 500 to 10,000 hxe2x88x921, preferably 800 to 8,000 hxe2x88x921. The reaction temperature is 250 to 550xc2x0 C., preferably 300 to 450xc2x0 C. The reaction can be carried out in either of under normal pressure and under imposed pressure, but usually under normal pressure.